The parent application shows backflushing in diffuser/thickener screen assemblies which is useful in a wide variety of vessels. The present invention relates to a specific improvement primarily for pressure diffusers.
Conventional pressure diffusers have a number of significant operating drawbacks. Because of the inlet geometry, the utilization of straight screens, and other design elements which are dictated by the pressurized environment, conventional pressure diffusers have a tendency to plug, shutting down completely and sometimes also disrupting other treatment vessels that are connected to it.
According to the present invention, a multiscreen pressure diffuser is provided which is much less likely to plug than conventional pressure diffusers. The multiscreen pressure diffuser according to the invention has design elements similar to conventional unpressurized diffusers, such as illustrated in the parent application drawings, while operating under pressurized conditions. The extraction liquid, treating liquid, and pulp flows are such that the screens are periodically backflushed without causing a surge in the vessel, and allow for smooth transport of the various elements to the desired areas. There are very few projections upon which the pulp can "hand up", and for any such projections liquid injection means are provided for injecting additional amounts of liquid so that the pulp at such points becomes more fluid and will not hang up.
According to one aspect of the present invention, a pressure diffuser comprises a generally vertical pressurizable liquid-tight vessel having a slurry inlet (preferably adjacent the bottom) and a slurry outlet (preferably adjacent the top). At least one ring-shaped screen element, providing first and second concentric screen surfaces, is mounted generally vertically within the vessel. A plurality of radially extending header arms mount the screen element for movement in the vessel and are in fluid communication therewith. A first linear actuator means is mounted in vertical alignment with the vessel, and is operatively connected to the arms for effecting reciprocation thereof. A cylinder, open at both ends, is affixed to the arms and extends above and below them to define a generally vertical pathway. A piston is disposed within the cylinder generally below the arms for reciprocation therewithin. A second linear actuator is disposed in vertical alignment with the vessel and means are provided for operatively connecting the piston to the second actuator to effect reciprocation of the piston and the cylinder. A guiding conduit surrounds an open end of the cylinder above the arms, for guiding reciprocation of the cylinder with the arms and for transporting liquid from the arms. An effluent outlet is connected to the guiding conduit for carrying liquid effluent from the guiding conduit to a location remote from the vessel. Treatment liquid is introduced into the vessel from a perforated jacket surrounding the guiding conduit, and from nozzles at the vessel walls. A second actuator is located above the arms and the connecting means comprises a plurality of rods arcuately spaced from each other and from the arms and extending above the arms to below the arms. Liquid injecting means are provided for injecting liquid beneath any stationary obstructions within the vessel to prevent slurry particles from collecting thereat.
The invention is primarily useful for the treatment of comminuted cellulosic fibrous material slurries (paper pulp). According to the method of the present invention, utilizing the pressure diffuser of the invention, the following steps are practiced: (a) Pressurizing the vessel. (b) Feeding slurry into the vessel under pressure to cause it to move past the screen surfaces and effluent liquid from the slurry to pass through the screen surfaces to flow to the header arms and to be discharged therefrom. (c) Removing the effluent liquid from the vessel. (d) Reciprocating the arms, with screen element, up and down. And, (e) periodically effecting back flushing of the screen surfaces without causing a surge within the vessel. There typically would be the further step of introducing treatment liquid into the vessel to pass through the slurry and displace at least some effluent liquid. Also the slurry is agitated, preferably at the top of the vessel, to facilitate smooth passage of the slurry particles out of the vessel after treatment.
In the pressure diffuser according to the invention it is desirable to provide the ring-shaped screen element with a pair of radially spaced ring-shaped plates disposed between the screen surfaces thereof and concentric therewith. Means are provided defining a plurality of pressure difference holes spaced along the length of the plates, and a solid wall portion of the ring-shaped element connects to the arms, such portion having a solid top wall with means defining a plurality of bores therewithin, with the bores disposed between the plates. While such a construction is particularly useful for a pressure diffuser, it may also be applied to other conventional diffusers that are not pressurized.
According to another aspect of the present invention a pulp treating apparatus in general is provided. That apparatus comprises: A generally vertical vessel having vertically spaced pulp inlet and outlet. A plurality of radially extending header arms for conducting fluid. First linear actuator means mounted in vertical alignment with the vessel and operatively connected to the arms for effecting reciprocation thereof. A cylinder, open at both ends, affixed to the arms and extending both above and below the arms to define a generally vertical pathway. A piston disposed within the cylinder generally below the arms for reciprocation therewithin. Second linear actuator means disposed in vertical alignment with the vessel and operatively connected to the piston to effect reciprocation of the piston in the cylinder. A guiding conduit surrounding an open end of the cylinder generally above the arms, for guiding reciprocation of the cylinder with the arms and for transporting liquid from the arms, and an effluent outlet connected to the guiding conduit for carrying effluent liquid from the guiding conduit to a location remote from the vessel.
It is the primary object of the present invention to provide for the effective treatment of slurries, such as comminuted cellulosic fibrous material slurries, and most desirably under pressurized conditions. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.